1. Field of the Invention
This invention pertains to the field of total knee endoprosthesis devices.
2. Description of the Prior Art
The human knee joint is one of the most stressed joints of the human body. In normal activities such as walking, running, kneeling, and climbing stairs, the load put on the knee joint can easily exceed five times the weight of the body, and for those who engage in more strenuous activities, can be much higher. Various forms of arthritis affect approximately 10 percent of the world's population. A significant number of those suffering from arthritis experience severe deterioration and deformity of joints such as the knee joint. Those who have experienced knee trauma are particularly susceptible to disabling arthritis. Approximately 140,000 patients are operated on each year in the United States alone for total knee replacement. Many more individuals suffering from knee joint problems would be candidates for total knee replacements if there existed knee endoprosthesis which more closely mimic the anatomical knee.
Knee joint prostheses can be classified into two basic types. The first type, referred to as "stabilized" prothesis, has hinge or ball and socket type joints used as substitutes for the anatomical knee joint. In this type of joint, the movement of the joint is constrained by a hinge pin or ball and socket. The stabilized prothesis is useful where little reliance can be placed on the surrounding soft tissues (i.e. tendons and ligaments) to stabilize the joint. Unlike the anatomical knee joint, these joints allow little, if any, anterior-posterior translation, lateral angulation, or rotation. For this reason, such joints are undesirable.
The second type of knee joint protheses are the so-called "condylar surface" protheses. In these types of protheses, the corresponding bearing surfaces on the femur and tibia are replaced by analogously shaped and positioned prosthetic bearing surfaces. Condylar surface prothesis joints rely upon the surrounding tendons and ligaments to hold the femoral and tibial portions of the joint, which are not directly connected to each other, together and to impart stability to the joint during movement. These types of joints are relatively compact and light, afford substantial rotational and translational freedom of movement, and require relatively minor resection of the natural bone and disturbances of the surrounding soft tissues during implantation. Examples of these types of prosthetic joints are disclosed in U.S. Pat. No. 3,798,679 to Ewald; U.S. Pat. No. 4,064,568 to Grundei, et al.; U.S. Pat. No. 4,224,696 to Murray, et al.; U.S. Pat. No. 4,673,408 to Grobbelaar; U.S. Pat. No. 4,714,472 to Averill, et al.; U.S. Pat. No. 4,770,663 to Hanslik, et al.; U.S. Pat. No. 4,822,365 to Walker, et al.; U.S. Pat. No. 4,822,366 to Bolesky; U.S. Pat. No. 4,944,756 to Kenna; U.S. Pat. No. 4,959,071 to Brown, et al.; and U.S. Pat. No. 4,985,037 to Petersen.
However, regardless of the type of artificial knee joint design, the medical community and designers of total knee endoprosthetic devices have long believed that the movement of the knee joint in flexion and extension does not take place in a simple hinge-like manner. Rather, it has heretofore been universally accepted that the flexion and extension knee movement includes displacement and rotation, so that the same parts of a first articulating surface (the condyles of the femur) are not always in contact with the same part of a second corresponding articulating surface (the tibial plateau) with the axis of motion not being fixed. Thus, it has been believed that the knee acts not as a simple hinge joint, but turns in extension and flexion through an infinite succession of uniquely placed centers of rotation, each center acting at a particular relative orientation of the femur and the tibia. See for example, "The Surgical Replacement of the Human Knee Joint," by David A. Sonstegard, et al., Scientific American, January, 1978, Vol. 238, No. 1.
U.S. Pat. No. 4,822,365 to Walker, et al. discusses the historical development of knee protheses design and provides general background in this area. Through the years, improvements have been made in the materials used to manufacture knee protheses and in the designs based on the belief that the flexion-extension axis is not fixed. However, as will be discussed in greater detail below, the inventor has discovered that in the anatomical knee, the flexion-extension axis is actually fixed. Because the designs of prior art total knee endoprostheses devices are based upon the incorrect basic assumption that the flexion-extension axis is non-fixed and has a constantly changing axis of rotation, the prior art prosthetic knee joints remain far inferior to the natural human knee in the degree of motion possible and the strains and stress put on these artificial knee joints. Although the artificial knee joints of today are far better than those of years past, they still suffer from failure by fracturing and detachment from the bones to which they are attached. These problems are in turn largely caused by the unnatural stresses and strains put on the prosthetic knee as a result of their designs.